extensive work on lighting, shadows, and fog

- use MeshLambertMaterial for interiors, terrain, and shapes
- use smooth vertex normal blending to avoid facted-looking contrasty lighting
  between adjacent surfaces
- update io_dif Blender addon to extract lightmaps
- re-export .dif files to glTF with lightmaps and without LOD
- enable sun, ensure correct direction
- adjust fog (more work to do)
- cleanup and optimization

src/terrainMaterial.ts

@@ -101,10 +101,14 @@ uniform float tiling4;
 uniform float tiling5;
 uniform float debugMode;
 ${visibilityMask ? "uniform sampler2D visibilityMask;" : ""}
-${detailTexture ? `uniform sampler2D detailTexture;
+${
+  detailTexture
+    ? `uniform sampler2D detailTexture;
 uniform float detailTiling;
 uniform float detailFadeDistance;
-varying vec3 vTerrainWorldPos;` : ""}
+varying vec3 vTerrainWorldPos;`
+    : ""
+}
 
 // Wireframe edge detection for debug mode
 float getWireframe(vec2 uv, float gridSize, float lineWidth) {